1. Field of the Invention
The present invention relates to a glass clear heavy gauge sheet with copolyester which has excellent transparency and all properties with cheap production cost.
Polyethyleneterephthalate (hereinafter called "PET") resin having been utilized in PET bottle, garment fiber, film and other vast fields due to its excellence of properties, processing, and economical aspects. It has been radically increased to use the PET resin as packing material based on its excellent mechanical properties, chemical resistance and recyclability. Sheet is one prevalently used among other fields, especially thin sheet(thinner than 1 mm) for vacuum forming. The polyester sheet thicker than 1 mm, however, is not easy for producing due to its crystallization and melt viscosity.
2. Description of Prior Art
In order to produce thick sheet(hereinafter called "thick sheet") with polymeric material, the polymeric material needs to have properties of low crystallization speed and low crystallization degree with proper high melting viscosity in order to maintain forming stability in melting condition. A new trial for the purpose of satisfying these properties, was to produce a modified polyester by copolymerization.
There are two methods in producing the modified polyester as follow:
First of all, substitute the some part of terephthalate moiety used as acid component during PET polymerization, such as Dimethylterephthalate (hereinafter called "DMT") or Terephthalic Acid (hereinafter called "TPA") with Diacid or Diacidester. As for Diacid or Diacidester, Isophthalic Acid (hereinafter called "IPA") or Dimethylisophthalate (hereinafter called "DMI") are applied mainly. This method, in other words, is producing polyester copolymer by polymerizing [i] DMT or TPA, [ii] DMI or EPA and [iii] Ethyleneglycol (hereinafter called "EG").
As expressed in following formula (I) and (II), TPA and IPA are same in their structure only except the position of functioning radical attached to Benzene chain. ##STR1##
[Formula (I) means TPA and formula (II) means IPA]
Since the reactivity of EPA in reaction circle during PET polymerization is fairly good, it reacts very well to some extent mol %. Since the copolyester, polymerized by some mol % of IPA or DMI, has the structure of formula (IV), it is different from homo PET which has the structure of formula (III). ##STR2##
According to the structural differences, the copolyester of formula (IV) has the effects of decreasing crystallization speed and crystallization degree since the interference to regularity of molecular chain during crystallization.
However, in proportion to the increase of EPA or DMI content, some properties of resin shall be suddenly deteriorated. In case, minimizing the IPA or DMI content for the purpose of preventing the deterioration of properties, it would not be possible to have the effects of sufficiently low crystallization rate. Because of this problem, the copolyester of formula (IV) constituted of around 3 mol % of EPA or DMI, is used only for producing thick bottle or thin sheet.
The second method is to substitute the some part of Ethyleneglycol used as one of the essential monomer applied during PET production with 1,4-Cyclohexandimethanol (hereinafter called "CHDM"). In other words, after preparing polyester copolymer by polymerizing [i] DMT or TPA, [ii] EG and [iii] CHDM, produce transparent and thick copolyester sheet by utilizing this polyester copolymer.
Since the length of CHDM molecule, having the structure of formula (V), is longer than that of EG, it disturbs the regularity of the molecule chain, and then decrease the crystallization rate and the crystallinity.
Furthermore, as the molecular chain of CHDM, like EPA, is not clamped, the reactivity and properties of polymer would not be deteriorated, and has high melt viscosity required for producing thick sheet even though the CHDM content increases ##STR3##
In order to obtain the properties, which enable to form the transparent and thick sheet, such as low crystallization rate, low crystallinity and high melt viscosity, it is necessary to use CHDM up to of 30 mol %. As a result, the producing cost comes to be very expensive and this is the limit in practical use.
Although, it is indispensable to have high molecular weight in order to obtain enough mechanical properties, if CHDM is polymerized in large quantity, there is no way except to raise molecular weight in the melt polymerization due to its amorphous phase by CHDM. Therefore, in order to obtain the high molecular weight, fairly good effort is needed to use with specially designed polymerization plant. Eastman Chemical Co. produces polyester copolymer by the second method, and sells them as common name "PETG" and also as trademark "SPECTAR".